Rescuers helping people after a boat carrying about 250 migrants from Libya crashed on to rocks last month as it tried to enter the port of Pantelleria, an island off the southern coast of Italy. Photograph: Francesco Malavolta/AFP/Getty Images

The Libyan regime of Colonel Muammar Gaddafi is allowing thousands of sub-Saharan African migrants on to overcrowded, unseaworthy ships in an apparently calculated attempt to use migration as a weapon to pressure Nato and the EU countries backing Libya's rebels.

Libyan officials admit they are not preventing boats full with African migrants embarking on perilous journeys to Europe. This is in protest at air strikes, which they say have destroyed the country's coastguard.

Turning a blind eye to people smuggling has had disastrous effects, apparently leading to the deaths of hundreds of boat people during unsuccessful attempts to reach the Italian island of Lampedusa, Malta, and other parts of Europe.

Officials are still assessing the death toll from the sinking of an unseaworthy and overcrowded ship that is feared to have claimed hundreds of lives when it went down less than two miles off Tripoli last Friday.

Officials said they were doing nothing to encourage the journeys to Italy, but could see no reason to stop them, because doing so would serve the interests of Nato member states bombing Libya.

"We say to Europe that we can no longer do what we used to do," said the prime minister, Al-Baghdadi Ali al-Mahmoudi. "And that's because Nato has ruined our coastal defences."

A senior UN official said there had been reports that some migrants had been ordered at gunpoint to board boats by the Libyan army.

Melissa Fleming, chief spokesperson for the United Nations high commissioner for refugees, said the agency had received reports that the Libyan army was organising and directing the exodus with scant regard for human life or for Libya's international obligations.

Fleming said one survivor had told the UN refugee agency that he and others had initially refused to board a ship they believed to be dangerously overcrowded. Libyan soldiers fired their guns into the air to force people to go on board, the survivor said.

Other survivors told the UN that some ships were leaving Tripoli only for their captain to disembark once they were at sea and take a pilot boat back to shore. "They [the migrants] are told, 'here's the compass, you go that way'," Fleming said.

However, survivors of Friday's tragedy told the Guardian they did not believe Libyan soldiers or officials had been involved.

They said smugglers came to their compounds in the capital at dawn to collect them. Two Somalis said they were stripped of their valuables by Libyan men they did not believe were officials. They were then taken to a beach 15 minutes east of the Tripoli port, where they were crammed into an 18-metre (59ft) boat.

The Guardian also spoke to survivors from Mali and Eritrea, who were among about 200 people who swam to shore or were rescued by the Libyan navy.

The nine men said they had not seen Libyan officials until they were rescued. They said they had paid $300-$500 (£180-£305) for the journey, less than half the fee being charged by smugglers a year ago.

"There were 885 people on the boat," said Mohammed Abdul Aziz. "It started swaying and we ran to one side and then we ran to the other. It sank and broke apart. We did not know where we were going."

Abdulghani Waeis, the chargé d'affaires at the Somali embassy in Tripoli, who is dealing with up to 150 fatalities and about 50 survivors from Friday's sinking, said: "There were efforts to prevent this last year. But now the Europeans are attacking Libya and you cannot defend those who are attacking you. It is a policy of not defending [the coast] but not pushing them [to sea]."

According to UN figures, 12,360 migrants from Libya have arrived in Italy and Malta since March and the figure is rising fast. Five boats carrying a total of 2,400 people, including many women and children, arrived off Lampedusa at the weekend.

About 10% of the boat people are believed to have perished during the voyages, either from drowning, exposure or a lack of food and water. Most of the people now leaving Libya are understood to be of sub-Saharan origin, including Somalis and Eritreans who were working in Libya before the war began earlier this year. Under a previous agreement with the EU, Gaddafi prevented them and other economic migrants from countries such as Niger and Chad from trying to travel on to Europe.

Italy says about 25,000 illegal immigrants, mostly from Tunisia, have arrived on its territory since the start of the year, while France is leading efforts to suspend the EU's internal open borders policy to halt migrant flows.

Cecilia Malmström, EU commissioner for home affairs, has demanded urgent action to address the deteriorating humanitarian situation in Libya. "Reports of men, women and children from sub-Saharan Africa being forced out of the country by the Gaddafi regime are particularly worrying," she said in a statement.

Last night Libyan state TV showed footage of an apparently healthy Gaddafi meeting officials in a Tripoli hotel on Wednesday, ending nearly two weeks of doubt over his fate since a Nato air strike killed his son.

Gaddafi, who had not appeared in public since the 30 April strike on his Bab al-Aziziyah compound killed his youngest son and three of his grandchildren, appeared in his trademark brown robe, dark sunglasses and black hat.

"We tell the world: 'those are the representatives of the Libyan tribes,'" Gaddafi said as he pointed to his visitors and then named a few of them.

An old man then told him: "You will be victorious."

The UN has called on European nations to urgently improve mechanisms for rescue at sea following revelations by the Guardian that 61 African migrants died in March after European military units, including a Nato ship, allegedly ignored their cries for help.